NEWTON — Olga Weisfeiler busied herself in the small kitchen of her ground floor apartment one recent evening, preparing a supper of soup, salmon, and grilled potatoes.

But as with all recent meals, she ate at the counter, not the kitchen table, which was blanketed with copies of newspaper articles, letters, e-mails, and recently declassified diplomatic cables she has been organizing for her next mission.

That mission began Saturday, when the 69-year-old part-time nanny left Logan International Airport for her 13th trip to Chile, part of her three-decade quest to learn the fate of her brother, Boris. He is the only American on a list of 1,000 people who “disappeared” under the military dictatorship of General Augusto Pinochet and whose fate remains unknown.

This time Weisfeiler is armed with new hope: her former senator, John F. Kerry, who has taken a personal interest in the case, is now the nation’s top diplomat. And she is appealing to him to help pressure US and Chilean authorities to open still-secret records.

She wants Chile to bring to justice the security officials who many believe kidnapped and murdered Boris Weisfeiler, a math professor who was 43 when last seen hiking near the Argentine border in 1985.

Wendy Maeda/Globe Staff

Olga Weisfeiler has been trying for decades to learn the fate of her brother, Boris.

“I just cannot give up,” Olga Weisfeiler, who followed her brother to the United States from Russia, insisted in her accented English, her bright green eyes welling with tears. “It is not about love. It is my responsibility, a responsibility to him and my conscience.”

This week she will meet with the US ambassador to Chile, Alejandro Wolff, as well as a Chilean judge, to press her case.

“I am going each year to push it forward,” she explained. “Otherwise it will stall.”

The fate of Boris Weisfeiler remains a mystery. Her conversations with local officials and US Embassy staff, the findings of a private investigator she hired in Chile, and bits of the official record Olga Weisfeiler has pried out of US files have not answered key questions.

Was Boris mistaken for a spy by Pinochet’s security forces? Did he stumble upon a nearby compound of escaped Nazis? Was he killed immediately, or kept prisoner for a time? Could he possibily still be alive?

Documents show in the months after he disappeared, the US State Department questioned the local authorities’ assertion that he died in a hiking accident. US officials raised the possibility that he had been detained by army or police forces and was being held prisoner in the Andean foothills in a compound known as Colonia Dignidad, where an escaped Nazi ran what a Chilean congressional report later described as a “state within a state.”

Peter Kornbluh, a researcher at the National Security Archive at George Washington University and an expert on Pinochet’s reign of terror, believes there are likely more American documents — particularly in the custody of the CIA — and Chilean government records that could help Olga Weisfeiler find the closure she seeks.

“Until every record of this time period, from every official who had some connection to this case, is searched for and reviewed and released, the family will never believe all has been done to find what happened to Boris Weisfeiler,” said Kornbluh.

“We haven’t even begun to see Chilean documents on this,” added Kornbluh, author of “The Pinochet File: A Declassified Dossier on Atrocity and Accountability,” a study of the 3,000 people believed killed or disappeared during Pinochet’s reign between 1973 and 1990.

Some of the pieces of the puzzle are piled neatly on Olga Weisfeiler’s kitchen table as she prepares for her meetings in Santiago.

A cable from a CIA officer assigned to the American embassy in Santiago in the months after her brother disappeared suggested there was information that he “was still alive.” A State Department memo a few months later stated that “the Chilean judge in charge of the investigation believes there may be another, more sinister explanation for Weisfeiler’s disappearance. So does the embassy.”

Other declassified US documents suggest that Boris Weisfeiler was not a victim of an accident but was in the custody of Chilean security officials. Even the Mathematics Society of Chile, acting on behalf of one of their colleagues, tried to find answers, concluding that if Weisfeiler was murdered, “it was done by professionals who do not operate in the usual way of Chilean criminals, and that the motive was not theft since the objects found in his backpack were very valuable.”

A breakthrough came last year with word that a Chilean judge, citing the evidence from US files, ordered the arrest of eight former police and army officials for kidnapping Weisfeiler and then covering it up. But no one has been arrested and no more information has been revealed.

The US Embassy in Santiago insists it is not giving up, saying that the Weisfeiler case is “a top human rights priority for the US government.”

“The US government has followed the Weisfeiler case for many years,” said Gabrielle Guimond
, the embassy spokeswoman in Chile. “We continue to work closely with the family and Chilean officials, and will continue to support the ongoing judicial investigation in Chile to support a just resolution of this case.”

Olga Weisfeiler is now counting on Kerry to make the difference.

“You are now in a position to make this happen and assist my quest to determine my brother’s fate, and my dream to find him, and find closure on this horrible human rights crime,” she wrote in a letter to him last month. She requested declassification of US documents, citing the possibility that her brother was alive more than a year after his disappearance.

Kerry did not respond directly to the request to release more documents. In a statement sent to the Globe Friday, he compared Weisfeiler’s refusal to give up to the families of soldiers missing in Vietnam.

“I see the same passion in Olga. It takes a moral fortitude and incredible character to go back to Chile 13 times to find answers and refuse to quit,’’ he said. “It’s also important in the context of finding closure regarding a tragic era in Chile’s history,’’ he said.

Former representative Barney Frank, a Newton Democrat who took up Weisfeiler’s case after she approached him during religious services at Temple Emanuel in 2000, believes more can be done.

“We kept raising the issue,” he said. “We complained to the Chileans. We complained to the State Department. We never got very far.”

In Frank’s view Weisfeiler “needs to know she has done everything possible. And for her to say that, I had to do everything I could.”

Weisfeiler is not giving up.

“I never had a feeling Boris died,” she said as she flipped through old photo albums and pointed at her smiling brother, who she said was always finding ways to help people — like the time, she recalled animatedly, when he ran after a train to help a tardy passenger to get aboard with her luggage.

There are pictures of him on his other travels to places like China and India. One of him proudly becoming a US citizen in 1981. And the last time she saw him, in Budapest in 1984.

“She has literally devoted her life to finding her brother,” said Kornbluh. “And I think there are Chilean Army and police officials who know exactly where he is.”